Photo: Jennifer Sinco Kelleher/AP - Women's Correctional Community Center inmate Lilian Hussein checks on ti leaves she planted as part of the prison's farming and gardening program in Kailua, Hawaii. The green ti leaves are often used to wrap food or weave into leis.

By Kristofor Husted for NPR

If you haven’t noticed, gardens are popping up in some unconventional places – from prison yards to retirement and veteran homes to programs for troubled youth.

Most are handy sources of fresh and local food, but increasingly they’re also an extension of therapy for people with mental health issues, such as post-traumatic stress disorder, or PTSD; depression; and anxiety.

It’s called horticultural therapy. And some doctors, psychologists and occupational therapists are now at work to test whether building, planting, and harvesting a garden can be a therapeutic process in its own right.

Much of the science behind just how gardening affects the mind and brain still remains a mystery. What scientists do know is that gardening reduces stress and calms the nerves. It decreases cortisol, a hormone that plays a role in stress response. So what about the biological mechanism behind mental disorders? That’s a bit tougher.

Variables in the environment — such as climate, location, diet and genetics — have complicated some of the early research on horticultural therapy. So to pinpoint a causal relationship between gardening and mental rehabilitation, researchers have to use a balance of qualitative and quantitative studies, according to Elizabeth Diehl, editor-in-chief of the Journal of Therapeutic Horticulture.

A 2011 study at a juvenile rehabilitation center in southwestern Ohio with a gardening program showed that horticulture therapy helped the kids see themselves in a more positive light and helped them better manage their emotional and behavioral problems.

To read the rest of Kristofor Husted’s story on the NPR website, click here.